Practical activity

Paper chromatography of ink

There are several ways that you could investigate the composition of inks. This is an outline of the required steps to undertake one of these methods.

Aim

To identify the mixture of dyes in a sample of unknown composition.

A pencil line is drawn across a sheet of chromatography paper and spots of ink or plant dye are placed along it. The paper is held abovea basin containing solvent.

Method

  1. Draw a pencil line across the chromatography paper, 1 to 2 cm from the bottom
  2. use a pipette or capillary tube to add a small spot of the unknown ink to the line on the paper
  3. add dot of a range of dyes to the paper
  4. place the paper into a container with a suitable solvent in the bottom
  5. allow the solvent to move through the paper, but remove the chromatogram before the solvent reaches the top
  6. mark the distance that the solvent has travelled
  7. then measure the distance travelled by each spot and by the solvent

Results

Record your results in a suitable table. For example:

InkSpot colourDistance travelled by spot (mm)

Analysis

  1. calculate the Rf value of each spot: Rf = \frac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent}
  2. compare the Rf values and colours of each spot in the inks and describe their similarities and differences
  3. compare the Rf values of the spots from the unknown ink with the Rf values of the dyes

Evaluation

Question

Explain why the distances travelled by each spot were measured in mm, rather than in cm.

Measurements in millimetres are more precise than those in centimetres. They will give more precise Rf values.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances. Eye protection must be worn.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardPossible harmPossible precaution
Harmful solventSkin irritationAvoid skin contact, eg wear gloves
Harmful solventBreathing difficultiesEnsure adequate ventilation or use a fume cupboard